Over-voltage protected semiconductor device fabrication

ABSTRACT

In accordance with the principles of the invention, a semiconductor substrate is provided that has a first cell formed thereon. The first cell has first and second terminals or nodes and a control terminal or node and has a characteristic breakdown voltage across the first and second terminals. A voltage sensing transistor is coupled across the power transistor first and second terminals. The voltage sensing transistor has a second element characteristic breakdown voltage that is less than the first cell characteristic breakdown voltage. The voltage sensing transistor provides a control signal to the terminal when the voltage across the first and second terminals exceeds the second element characteristic breakdown voltage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to co-pending application Serial No. ______ filed on even date herewith and assigned to a common assignee.

FIELD OF THE INVENTION

The invention pertains to semiconductor devices, in general, and to providing over-voltage protection to semiconductor devices, in particular

BACKGROUND OF THE INVENTION

Inductive loads switched by a power transistor can produce voltages high enough so that without over-voltage protection, the power transistor may be permanently damaged.

In the past, one way of providing protection for power MOSFETs has been utilizing a feedback path comprising a series connected zener diode and a conventional diode connected across the gate-drain of the MOSFET as shown in FIG. 1.

To protect against over-voltage, the zener voltage Vz plus the diode drop voltage Vd plus the MOSFET gate to source voltage Vgs must be less than the MOSFET breakdown voltage. As a practical matter, the total voltage drop of Vz+Vd+Vgs must be much less than the MOSFET breakdown voltage due to the fact that these elements do not track fabrication process variations in the devices.

It is desirable to provide a power transistor device that has over-voltage protection integral to the device.

SUMMARY OF THE INVENTION

In accordance with the principles of the invention, a semiconductor device is provided that comprises a substrate that has a first circuit cell formed thereon and a voltage sensing cell also formed on the substrate. The first circuit cell has first and second nodes and has a first characteristic breakdown voltage across the first and second nodes. The first characteristic breakdown voltage is determined by the geometric layout of the first circuit cell. The voltage sensing cell comprises a transistor coupled across the first and second nodes and has a second element characteristic breakdown voltage. The sensing transistor has a geometric layout such that the second element characteristic breakdown voltage is determined by a second layout distance of the sense transistor.

When the voltage across the first circuit cell first and second terminals reaches the characteristic second element breakdown voltage, the voltage sensing transistor turns on and, in turn causes the first circuit cell to turn on protecting the first circuit cell from any increase in voltage.

The illustrative embodiment of the invention is a semiconductor device comprising a substrate having a power transistor formed thereon. The power transistor comprises first and second terminals and a control terminal and has a characteristic first breakdown voltage across the first and said second terminals. The power transistor has a geometric layout such that the characteristic first breakdown voltage is determined by a first layout distance of the power transistor. The semiconductor device also comprises a voltage sensing transistor formed on the substrate. The voltage sensing transistor is coupled across the first and second terminals and has a second element characteristic breakdown voltage that is less than the characteristic first breakdown voltage. The sensing transistor has a geometric layout such that the second element characteristic breakdown voltage is determined by a second layout distance of the sense transistor.

In accordance with an aspect of the invention the power transistor and the voltage sensing transistor are concurrently fabricated on the substrate. The power transistor is laid out on said substrate to have the characteristic first breakdown voltage, and the voltage sensing transistor is laid out on the substrate to have the second element characteristic breakdown voltage.

In the illustrative embodiment of the invention the power transistor comprises at least one MOSFET power transistor first cell having a source, a drain and a gate. The MOSFET power transistor first cell has a first drain to source characteristic breakdown voltage. The voltage sensing transistor comprises a MOSFET transistor second cell having a source, a drain connected in common with the MOSFET power transistor first cell drain, and a gate connected in common with the MOSFET power transistor first cell gate. The MOSFET transistor second cell is designed to have a drain-source characteristic breakdown voltage, also referred to herein as a characteristic second element breakdown voltage, that is less than the first characteristic breakdown voltage. The difference in breakdown voltages is determined by the geometric layout of the cells.

In the illustrative embodiment of the invention, the power transistor comprises at least a second MOSFET power transistor first cell that is substantially identical to the at least one MOSFET power transistor first cell.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be better understood from a reading of the following detailed description of the drawing figures in which like reference designations are utilized to identify like elements, and in which:

FIG. 1 illustrates a prior art arrangement for protection of a power MOSFET device;

FIG. 2 illustrates a MOSFET arrangement in accordance with the principles of the invention; and

FIG. 3 illustrates a second MOSFET arrangement in accordance with the principles of the invention;

FIG. 4 is a cross-section of one FET arrangement in accordance with the principles of the invention;

FIG. 5 is a cross-section of another FET arrangement in accordance with the principles of the invention;

FIG. 6 is a top planar view of a lateral diffused MOS FET arrangement in accordance with the principles of the invention; and

FIG. 7 is a top planar view of another MOS FET arrangement in accordance with the principles of the invention.

DETAILED DESCRIPTION

To obtain higher current switching capability, power transistors may be comprised of a plurality of individual power transistor cells fabricated on a substrate and connected in parallel.

Turning now to one illustrative embodiment of the invention shown in FIG. 2, MOSFET power transistor structure 1 is fabricated on a single substrate 201. The power transistor structure 1 includes two MOSFET power transistor cells T1, T2 fabricated on the single substrate 201. Transistor cells T1, T2 have their respective gates, drains and sources connected in common to gate node G, drain node D, and source node S, respectively. Each of transistor cells T1, T2 has a first characteristic drain-source voltage Vds which is a characteristic breakdown voltage that is dependent on several factors, including but not limited to the layout of the transistors on the substrate 201. The layout of both transistor cells T1, T2 is identical and the characteristic drain-source breakdown voltages of both cells are the same.

Although two transistor cells are shown, it will be appreciated by those skilled in the art that the power transistor structure may have fewer or more transistor cells.

In addition to the two power transistor cells T1, T2, power transistor structure 1 includes a breakdown voltage sensing transistor cell T3 is formed on substrate 201. Transistor cell T3 is fabricated at the same time as transistor cells T1, T2 but is designed to have a drain source characteristic breakdown voltage Vds that is less than the drain-source characteristic breakdown voltage of the power transistor formed by transistor cells T1, T2. Voltage sensing transistor cell T3 has its drain and gate connected to the drain D and gate G, respectively, of transistor cells T1, T2. The source terminal of transistor cell T3 is coupled to a current limiting device or resistor. In the illustrative embodiment of the invention, resistor R is formed on the same substrate 201, but may in some embodiments be separate from substrate 201.

In operation, when the drain-source voltage Vds across transistor cells T1, T2 reaches the second element characteristic breakdown voltage of sensing transistor cell T3, transistor cell T3 conducts current. The current through transistor T3 is limited by resistor R which prevents damage to transistor cell T3. A voltage is produced across resistor R at sense terminal S′. The voltage at sense terminal S′ may be utilized to control the gate of the power transistor formed by transistor cells T1, T2 such that the combined power transistor device formed on substrate 201 makes power transistor 1 self-protecting against breakdown voltages applied across its switching path.

The drain-source breakdown voltage of sense transistor cell T3 tracks the drain-source power transistor cells T1, T2 over process variations since it is an integral part of the power transistor structure 1.

In the illustrative embodiment shown in FIG. 2, transistor cells T1, T2, T3 are N-channel MOSFET structures. As will be appreciated by those skilled in the art, the principles of the invention may also be utilized with other transistor structures including P-channel MOSFET, N- and P-channel IGBTs, as well as NPN and PNP transistors.

Turning now to FIG. 3, a feedback circuit or path 301 is provided from the sense transistor cell T3 to the control or gate input G of the power transistor structure 1. Feedback circuit or path 301 includes an amplifier circuit 303. As the voltage across sensing element or transistor cell T3 reaches the second element characteristic breakdown voltage, sensing element or transistor cell T3 produces a voltage at sense terminal or node S′, amplifier 303 provides an output level at control input or gate G of the power transistor structure 1 to turn on the power transistor cells T1, T2 and sense cell T3 thus providing protection by limiting the applied drain voltage to less than the breakdown voltage of cells T1, T2.

The specific structure of amplifier 303 may be any one of a number of known feedback amplifiers. In addition a gate circuit may also be included to assure that the power transistor structure 1 does not turn on as power is applied.

In the illustrative embodiments of the over-voltage protected structures, the breakdown voltage sensing cell or element T3 is shown with a common drain to the power MOSFET cells or elements T1, T2, and the sense output is derived from the source of the over-voltage sensing element T3. In other embodiments, the voltage sensing element source is in common with the sources of the power MOSFET elements, and the sense output is derived from the drain of the breakdown voltage sensing element.

Monolithic power devices such as structure 1 shown in FIGS. 1-3 are generally comprised of a plurality of identical smaller cells such as transistor cells T1, T2.

As shown and described above one or more sense cells or transistors T3 embedded in an integrated multi-celled power device 1 are used to sense that the main power device is nearing breakdown its characteristic breakdown voltage.

Each sense cell T3 is configured to have a predefined breakdown voltage that is lower and tracks the breakdown voltage of the main power device 1.

In many types of semiconductor devices, for example CMOS and lateral diffused MOS (LDMOS), the geometric design layout of the device is one parameter that determines breakdown voltage.

In accordance with the principles of the invention a power device comprises a plurality of identical cells T1, T2, T3. One or more of the cells is designated as sense cell T1. The physical dimensions of the sense cell are modified from that of the other cells.

Turning now to FIGS. 4-7, the geometric design layouts of various monolithic power devices 1 including a sense element T3 are shown. Each of the planar layouts shows of FIGS. 4 and 5 shows a portion of a substrate on which FET transistor cells are formed. Power transistor cell T2 has a source finger 403. Sense cell transistor has a source finger 409. Both transistors T2, T3 utilize the substrate to form a drain 401. Transistor 402 includes a metallization layer 405 as a gate and transistor 403 includes a metallization layer 411 as its gate. The geometric layout of transistors T2, T3 determines the respective source-drain breakdown voltages. Transistor T2 has a characteristic source drain breakdown voltage that is determined, in part, by the distances X and X′ between finger 402 and the drain or substrate 401. Transistor cell T3 has a second element source-drain characteristic breakdown voltage that is determined by the distances Y and Y.′ At least one of the distances Y and Y′ is less than the corresponding distance X and X′, respectively. The smaller the distance Y is than X or Y′ is than X′, the lower the characteristic breakdown voltage of Transistor cell T3 is compared to the characteristic breakdown voltage of Transistor cell T2.

FIGS. 6 and 7 illustrate different LDMOS and MOS devices each having at least one power cell transistor T2 and a sense cell transistor T3. The geometric layout of transistors T2 and T3 again have different distances X and Y between the respective source and drain. Distance Y is chosen to provide a characteristic source-drain breakdown voltage for transistor cell T3 that is less than the characteristic source-drain breakdown voltage of transistor cell T2.

FIG. 5 shows another geometric layout of a sense transistor T3 in which the source finger 409 terminates in a point 505. The corresponding end surface 507 of drain 401 includes a point 509 disposed opposite point 505. The opposing points 505, 509 define a distance Y′ that is less than the corresponding distance X′ of transistor cell T2. The distance Y′ determines the characteristic source drain breakdown voltage of transistor cell T3.

The invention has been described in terms of specific embodiments. It will be appreciated by those skilled in the art that various changes and modifications may be made to the embodiments without departing from the spirit or scope of the invention. It is intended that the scope of the invention not be limited to the specific embodiments shown and described, but that the scope of the invention be limited only by the claims appended hereto. 

1. A semiconductor device comprising: a substrate; a power transistor formed on said substrate, said power transistor comprising first and second terminals and a control terminal and having a characteristic first breakdown voltage across said first and said second terminals, said power transistor having a geometric layout such that said characteristic first breakdown voltage is determined by a first layout distance of said power transistor; a voltage sensing transistor formed on said substrate, said voltage sensing transistor being coupled across said first and second terminals, said voltage sensing transistor having a second element characteristic breakdown voltage, said second element characteristic breakdown voltage being less than said characteristic first breakdown voltage, said sensing transistor having a geometric layout such that said second element characteristic breakdown voltage is determined by a second layout distance of said sense transistor.
 2. A semiconductor device in accordance with claim 1, wherein: said power transistor and said voltage sensing transistor are fabricated concurrently on said substrate.
 3. A semiconductor device in accordance with claim 1, wherein: said power transistor comprises a first FET and said first layout distance is a source to drain distance of said first FET; and said sensing transistor comprises another FET and said second layout distance is a source to drain distance of said another FET.
 4. A semiconductor device in accordance with claim 3, wherein: said power transistor comprises a second FET, said second FET being geometrically identical to said first FET.
 5. A semiconductor device in accordance with claim 1, wherein: said power transistor comprises at least a first and a second transistor cell, said first and said second transistor cells having substantially identical geometric layouts on said substrate.
 6. A semiconductor device in accordance with claim 6, wherein: said sensing transistor has a geometric layout that differs from the geometric layout of said first and second transistor cells such that said sensing transistor has a breakdown voltage that is less than a corresponding breakdown voltage of said first and second transistor cells.
 7. A semiconductor device in accordance with claim 6, wherein: said sensing transistor geometric layout includes opposing pointed configurations separated by said second distance.
 8. A semiconductor device in accordance with claim 1, wherein: said power transistor and said voltage sensing transistor each comprise MOS devices.
 9. A semiconductor device in accordance with claim 1, wherein: said power transistor and said voltage sensing transistor each comprise LDMOS devices.
 10. A semiconductor device comprising: a substrate; a first circuit cell formed on said substrate, said first circuit cell having first and second nodes, said first circuit cell having a first characteristic breakdown voltage across said first and second nodes, said first characteristic breakdown voltage being determined by the geometric layout of said first circuit cell; a voltage sensing cell formed on said substrate, said voltage sensing cell comprising a transistor coupled across said first and second nodes, said voltage sensing transistor having a second element characteristic breakdown voltage, said sensing transistor having a geometric layout such that said second element characteristic breakdown voltage is determined by a second layout distance of said sense transistor.
 11. A semiconductor device in accordance with claim 10, wherein: said first characteristic breakdown voltage is determined by a first distance between first elements of said first circuit cell; and said second element characteristic breakdown voltage is determined by a second distance between elements of said voltage sensing transistor, said second distance is less than said first distance.
 12. A power semiconductor device, comprising: a semiconductor substrate: said semiconductor substrate comprising at least one MOS power transistor first cell, said at least one MOS power transistor first cell comprising a source, a drain and a gate, said at least one MOS power transistor first cell being fabricated in said substrate to have a drain to source characteristic first breakdown voltage determined by a first layout distance between said drain and said source; said semiconductor substrate further comprising a MOS transistor second cell, said MOS transistor second cell having a source, a drain connected in common with said at least one MOS power transistor first cell drain, and a gate connected in common with said at least one MOS power transistor first cell gate, and said MOS transistor second cell being fabricated in said substrate to have a designed to have a drain to source characteristic second breakdown voltage determined by a second layout distance between said transistor second cell drain and said transistor second cell source, said second layout distance being less than said first distance.
 13. A power semiconductor device in accordance with claim 12, wherein: said semiconductor substrate comprises at least a second MOS power transistor first cell, said second MOS power transistor first cell being substantially identical in layout to said at least one MOS power transistor first cell, said at least a second MOS power transistor first cell having a drain connected in common with said at least one MOS power transistor first cell drain and said MOS transistor second cell drain, said at least a second MOS power transistor first cell having a gate connected in common with said at least one MOS power transistor first cell gate and said MOS transistor second cell gate; said at least a second MOS power transistor first cell having a source connected in common with said at least one MOS power transistor first cell source.
 13. A power semiconductor device, comprising: a semiconductor substrate: said semiconductor substrate comprising at least one power transistor first cell, said at least one power transistor first cell comprising a first terminal, a second terminal and a control terminal, said at least one power transistor having a first characteristic breakdown voltage between said first and said second terminals; said semiconductor substrate further comprising a transistor second cell, said transistor second cell having a first terminal connected in common with said at least one power transistor first cell first terminal, and a control terminal connected in common with said at least one power transistor first cell control terminal, and said transistor second cell being designed to have a second element characteristic breakdown voltage, said second element characteristic breakdown voltage being less than said first breakdown voltage. 